Exobasidium leaf and fruit spot emerged as a serious disease of blueberry in commercial production areas in the last decade. However, the fungal pathogen that causes the disease, Exobasidium maculosum, has high genetic diversity and population genetic structure indicating it is well established. The aim of this research was to investigate whether vectors play a role in transmission of the disease and identify factors that contribute to its high level of genetic diversity. Arthropod exclusion and trapping studies were conducted at two field sites in Alma and Toccoa, GA from 2018 to 2020. In each field, ten trap plants were covered with exclusion nets and paired with ten uncovered plants under diseased blueberry bushes for two weeks during the known infection window. The plants that were covered had 0 to 12 leaf spots while the uncovered plants developed up to 654, indicating that we excluded an agent necessary for infection to occur and points to vector transmission of the disease. A fluctuation assay that was performed to compare the spontaneous mutation rate of six E. maculosum strains and two Exobasidium rostrupii strains revealed that E. maculosum has a higher mutation rate. CHEF electrophoresis performed to assess karyotypes of 15 E. maculosum strains showed five chromosomes with sizes at approximately 3, 3.5 and 5 Mb, one larger than 5.7 Mb and one varying among strains between 2.35 and 2.7Mb, resulting in genome size estimates of 19.6 to 19.9Mb. Ploidy variation among 14 strains was assessed using flow cytometry and a bioinformatics approach and revealed that E. maculosum has ploidy and cell size variation and a high incidence of aneuploidy. Analysis of copy number variation revealed that there was an average of 500 deletions and 5 duplications among strains, and that duplications were greater in the northeast North America population. Phylogenetic relationships of 14 E. maculosum strains from Georgia, North Carolina and northeast North America were evaluated using whole genome sequence data. Strains clustered based on geographic location of origin. These findings are important in the epidemiology of the disease and highlights new factors that should be considered in its management.
